The cerebellum is a brain structure responsible for integrating and coordinating neurological inputs from several sources in order to maintain equilibrium and balance. It is an interesting brain structure in that it develops after birth. One of the cell types within the cerebellum that is critical for proper postnatal development are cerebellar granule neuron precursors (CGNP). During the first two weeks of life in mice these cells undergo a rapid expansion phase. Failure of CGNPs to exit this high proliferate program has been suggested to lead to medulloblastoma formation, the most common form of pediatric brain tumor. Medulloblastoma is relatively responsive to traditional cancer treatments, including surgery, chemotherapy and radiation. However long-term survivors often suffer from life-long developmental, behavioral, and cognitive disturbances. These devastating side effects underscore the need to understand the basic mechanisms underlying medulloblastoma, so novel treatments can be developed to specifically target tumor cells without damaging the developing brain. The long-term objectives of this project is to identify nodes of cross-talk between Sonic hedgehog and IGF signaling pathways both of which have been implicated in medulloblastoma. Greater understanding of these pathways may lead to the development of small molecule treatments reducing the need for radiation and chemotherapy in these young patients. In order to accomplish this goal we have developed an interdisciplinary proposal incorporating various techniques and models. Our preliminary data indicate that Shh signaling increases IRS1 protein levels, a scafold directly downstream of the IGF receptor, but not IRS1 mRNA in cultured CGNPs. In aim 1 we will test the hypothesis that Shh proliferative signaling in CGNPs positively regulates IGF pathway activity through up-regulation of IRS1 protein. We will look at IRS1 protein turnover as well as polysome analysis including microarray and mass spectrometry analysis. In aim 2 we will use retroviral mediated over-expression of IRS1 and lentivirally-delivered shRNAto determine if IRS1 is necessary/sufficient for Shh mediated CGNP proliferation in culture. In aim 3 we will determine the role of IRS1 in vivo by analyzing CGNP proliferation and cerebella development in mice lacking IRS1 either constitutively or conditionally in the cerebellum. In conjunction with MSKCC Mouse Genetics core facility we will develop mice conditionally lacking IRS1 in the cerebellum. We will analyze the role of CGNP proliferation using in vivo as well as in vitro by culturing CGNPs and treating with Cre to remove IRS1.